21 L=0n2 ()I 2 B B B L R I= RC2 1 22 21 2 dI dI d d L M dt dt dt dt    = − − = − − 2 L n =  0 1 2 ( ) 2 U Li u r dr B B = =  2 0 1 1 ( ) ( ) 2 B r u r B r   = L L R  =  c = RC